WASHINGTON — When a lonely Virginia teenager named Ali Amin got curious about the Islamic State last year and went online to learn more, he found a virtual community awaiting. It had its own peculiar language, stirring imagery and just the warm camaraderie, sense of adventure and devotion to a cause that were missing from his dull suburban life.

At 17, the precocious son of a Yemeni immigrant family, he quickly developed online relationships with older Islamic State supporters around the globe. There was Zubair in Britain, Uthman in South Africa and Abdullah in Finland, who urged him to start a Twitter account under the name AmreekiWitness, or American witness. Mr. Amin drew several thousand followers, sparred online with the State Department, engaged with prominent Islamic State propagandists and developed quite a name among English-speaking fans of the militants — until his arrest in February.

“For the first time, I felt I was not only being taken seriously about very important and weighty topics, but was actually being asked for guidance,” Mr. Amin wrote in August to the judge overseeing his case, expressing regret for what he portrayed as a disastrous youthful mistake. “By assimilating into the Internet world instead of the real world, I became absorbed in a ‘virtual’ struggle while disconnecting from what was real: my family, my life and my future.”

Photo: Ali Amin, a 17-year-old from Virginia, was sentenced to 11 years in prison after pleading guilty to material support for a terrorist group.

As the Obama administration takes on the multidimensional challenge posed by the Islamic State after the killings in San Bernardino, Calif., the online community of sympathizers in the United States is a critical focus. They number in the hundreds, experts say, and fit no single profile. Among those whose flirtations took a serious turn and led to criminal charges are a trio of teenage siblings from Chicago, a former Air Force mechanic in his late 40s from New Jersey, and a mother of two from Philadelphia.

In fact, they have little in common except one thing: the weeks or months spent marinating in the rhetoric and symbolism of the Islamic State, courtesy of Twitter and other Internet platforms.

It is in this electronic hothouse of mutual support, a sort of round-the-clock pep rally for a cause most Muslims shun, that Americans join other English speakers to try out defiant screen names, throw around Arabic words they have often just learned, and seek to outdo one another in pious zeal. Some merely express anger at American foreign policy or at what they see as mistreatment of Muslims overseas. Others go further, trying to reach Islamic State territory or plotting violence at home.

Like most heady American romances with the Islamic State, Mr. Amin’s came to a crashing halt. In late August, he was sentenced to 11 years in prison after pleading guilty to material support for a terrorist group. Americans who managed to reach Syria have suffered a still grimmer fate, dying on distant battlefields. And last week, in California, two admirers of the extremist group were shot dead by the police after attacking an office holiday gathering and killing 14 people.

Graphic: Where ISIS Has Directed and Inspired Attacks Around the World

The full story of the radicalization of the attackers, Syed Rizwan Farook and Tashfeen Malik, is still emerging as the F.B.I. retrieves records from deleted computer drives and smashed cellphones. But Ms. Malik’s decision, as the shooting began, to post on Facebook a pledge of loyalty to the leader of the Islamic State, also known as ISIS or ISIL, suggested that the couple had been exposed to the Internet world created by the group’s supporters and recruiters.

“It’s a closed community — almost a clique,” said Seamus Hughes, co-author of a report, “ISIS in America,” released last week by George Washington University’s Program on Extremism. “They share memes and inside jokes, terms and phrases you’d only know if you were a follower.”

The George Washington University report underscored the diversity of the 71 people in the United States charged with crimes related to the Islamic State since March 2014: 40 percent were converts to Islam, defying any ethnic profile. They were young, with an average age of 26; overwhelmingly American citizens or legal residents; and 14 percent were women.

But all, or nearly all, had spent hours on the Internet trumpeting their feelings about the Islamic State and engaging with English speakers from many other countries. In fact, nearly all were arrested after their online posts drew the attention of the F.B.I.

An Insular Community

Finding the Islamic State’s corner of the Internet is not hard. In March, Nader Saadeh of New Jersey performed simple online searches for the term “ISIS.” He read newspaper articles and looked at maps. Within weeks, he was downloading hourlong propaganda videos. At the end of April, prosecutors said, he bought a plane ticket to Jordan in hopes of getting to Syria to fight.

Curious web surfers can easily recognize the distinctive iconography that Islamic State supporters embrace. The black flag used by many jihadist groups, often inscribed in white with the Shahada, or Islamic creed, is popular. Portraits of jihadist heroes, notably the American Al Qaeda propagandist Anwar al-Awlaki, are regularly featured. Green birds, a symbol of paradise, and lions, a reference to warriors as “lions of Allah,” are favorites. Keonna Thomas, 30, a Philadelphia mother charged this year with trying to join the Islamic State, posted on Twitter as YoungLioness.

Photo: Calling himself @AmreekiWitness, or American witness, a Virginia teenager named Ali Amin posted a comment on Twitter after a police shooting in Ferguson, Mo., in August 2014.

Like political followers or sports fans, Islamic State supporters post videos and make memes to share with online friends. But the videos sometimes show beheadings, executions or angry lectures. And the photos declare their allegiance (“We are all ISIS”) or glorify young men with automatic weapons.

Even novices learn to lace their posts with transliterated Arabic: “kuffar” for non-Muslims; “shahid” for martyr; “khilafa” for the caliphate, the unified Muslim state that the group purports to be building. “It’s a shallow appropriation of things from an outside culture to demonstrate authenticity,” said Alberto M. Fernandez, a former State Department counter-radicalization official who is now at the Middle East Media Research Institute, and who wrote an essay on the insider language of Islamic State fans.

For more advanced users, there is “taghut,” a blanket epithet for opponents of jihad; “jizya,” a tax to be imposed on Christians and Jews who will not convert; and “baqiyah,” or “here to stay,” shorthand to announce Twitter users’ return after their accounts have been suspended.

Twitter remains the major gateway for those infatuated by the Islamic State, though YouTube, Facebook, Ask.fm, Tumblr, Instagram and other sites have a place in the group’s online ecosystem. As Twitter has more aggressively suspended the accounts of overt supporters, they have often migrated to Telegram, Kik, WhatsApp and other services that allow private communications, said Humera Khan, executive director of Muflehun, a nonprofit that studies radicalization and how to counter it.

Ms. Khan said that more sophisticated Islamic State promoters policed the public sites, circulating the screen names of people who had challenged the group’s claims and encouraging others to block their posts. The result, she said, “is an echo chamber where there’s a sense that no other view is legitimate.”

Photo: The Internet community created by Islamic State supporters and recruiters is tight-knit. Here, an account alleged to be from Miami retweeted another sympathizer.

The insular culture reinforces the idea that the United States is at war with Islam and portrays terrorist groups as nobly resisting America’s global military ambitions. The group’s arguments feed off American military action overseas, as well as anti-Muslim populism of the kind recently promoted by Donald J. Trump.

“Al-Qa’ida said it loud and clear: we are fighting the American invasion and their hegemony over the earth and the people,” Mufid A. Elfgeeh, a 30-year-old from Rochester wrote on Twitter before being charged last year with recruiting for the Islamic State. In another post, he predicted that the group would one day rule the world.

Calculated Recruitment

Into this world of Americans and English speakers from many other countries, the Islamic State’s propagandists regularly insert professionally produced material. Javier Lesaca, a visiting scholar at George Washington University’s School of Media and Public Affairs, said the group had produced some 1,015 videos since January 2014, including at least 37 in English aimed at recruiting foreign fighters.

The videos “are based and constructed according to Western values and images,” he said.

Their consistent message, Mr. Lesaca said, is that the Islamic State is a social movement devoted to protecting Muslims and fighting an unfair global economic system; that it does not discriminate on the basis of race or nationality; that it uses violence in self-defense and in ways that mimic Western films and video games; and that Westerners who join the fight in Syria and Iraq are normal people fighting a just war.
By Poh Si Teng and Ben Laffin 8:07
Flirting With the Islamic StateVideo
Flirting With the Islamic State
By Poh Si Teng and Ben Laffin on Publish Date June 27, 2015. Photo by Andrea Bruce for The New York Times. Watch in Times Video »

In fact, like other American followers of the Islamic State, Mr. Amin was drawn into the virtual jihadist world by his concern about atrocities being carried out against civilians by the Syrian president, Bashar al-Assad, according to court documents. He had Crohn’s disease, a chronic gastrointestinal disorder, which derailed a promising academic record and left him isolated.

Mr. Amin raised his concerns about Syria with several adults, including two imams and a Christian minister, his lawyer, Joseph Flood, said. None gave him much time or took his questions about Islam seriously. But on his laptop, alone in his bedroom in Manassas, Va., he found powerful emotional support for his militant pronouncements.

“He’s being told, ‘Not only are you smart and wise — you’re a leader,’ ” Mr. Flood said.

Soon, Mr. Amin, who displayed on his Twitter account an image of an Islamic State flag flying over the White House, was driving a friend to the airport so he could travel to Syria and posting an article on how to use the digital currency Bitcoin to support the jihadists.

The Internet had allowed him to develop “a secret, independent identity” without his family’s knowledge, Mr. Flood said. Mr. Amin adopted a stern, adult tone to instruct others in online security measures and he drew the State Department’s anti-Islamic State Twitter account, @ThinkAgainTurnAway, into an exchange. For an ailing teenager with a protective mother, it was an intoxicating time.

“Ali’s involvement in these activities came as a complete shock to everybody who knew him, because he was such a kind and gentle person,” Mr. Flood said.

Law enforcement officials say one advantage has been that many of those who go online to cheer on the Islamic State have been astonishingly indiscreet. Abdurasul Juraboev, 24, a Brooklyn man who worked in a gyro shop and was arrested in February, had posted on a pro-Islamic State website that he wished he could join the group abroad. But he wondered whether there were other ways to contribute, writing, “What I’m saying is, to shoot Obama and then get shot ourselves, will it do?” He lamented that he had no weapons.

A Kansas man, John T. Booker Jr., posted last year on Facebook: “I will soon be leaving you forever so goodbye! I’m going to wage jihad and hopes that i die.”

“Getting ready to be killed in jihad is a HUGE adrenaline rush!!” he wrote, according to court documents. “I am so nervous. NOT because I’m scared to die but I am eager to meet my lord.” He was arrested outside an Army base as he connected wires to what he thought was a bomb; it was a fake provided by an F.B.I. informant.

But such brazen displays of jihadist sentiment and public confessions to crimes are becoming less common, experts said. The steady string of arrests and the crackdown by social media companies are having an effect.

“The network of ISIS supporters is getting smaller, but tighter and more inward looking,” said Ms. Khan, the expert on countering extremism, adding that the San Bernardino attack may accelerate that trend by intensifying government scrutiny. That may shrink the Islamic State’s propaganda profile, but it will also make the group’s circles harder to penetrate and its supporters more difficult for law enforcement to track, she said.

But the initial reaction to the San Bernardino attack from English-speaking Islamic State supporters showed no such discretion, said Anat Agron, who tracks supporters for the Middle East Media Research Institute.

“People were rejoicing, celebrating,” Ms. Agron said. “They were talking about how beautiful it was for a married couple to do this together.”

Correction: December 9, 2015
An earlier version of this article misstated the timing of Ali Amin’s arrest. It was in February, not March.